Friday, January 18, 2013

The Different "Flavors" of Lake Erie Lake Effect Snow

A survey was done a few years ago about what weather words people latch onto when they heard them on television. The first was the word FORECAST, close to the top were the words LAKE EFFECT. Yet lake effect isn't a "one-size-fits-all" type of meteorological phenomena.

First the variables that go into a lake effect event. Some are independent of the lake (2 and 5 specifically) but work in concert with it.

1. Temperature difference between the lake and the air aloft (5000 feet) has to be at least 13 degrees celsius. The more, the better.2. Abundant atmospheric moisture independent of the lake3. Wind speed4. Wind direction5. Fetch Length (How far does the wind blow over the lake)6. Instability and instability depth (Usually driven by a cold front/trough and/or the lake temperature difference) Deeper the instability, the deeper the snow growth7. Orographic lift (elevation differences between Cleveland and the snowbelt. 1200 feet in portions of Geauga county)

Even if #1, 3, 4, 5 and 6 are present (the most common variables), subtract any one of these variables especially 2 and 5--sometimes 7 if the wind direction shifts) and snowfall forecasts can turn out much different than anticipated.

What are the different "flavors" of lake effect here in northern Ohio? Why does the secondary snowbelt vary so much? How can some areas like Akron get significant lake effect while the traditional snowbelt receives minimal amount? The differences lie in the wind direction.

The first example is what I like to call: CLASSIC LAKE EFFECT EFFECT SNOW.

Winds are primarily out of the northwest, usually behind an Alberta Clipper type cold front. The higher elevations east of Cleveland (Geauga, Lake and Ashtabula counties) AND SOUTHWEST OF CLEVELAND enhance the snowfall due to the lifting of the moist air. The secondary snowbelt in this example would include northern Medina, Summit and Portage counties. This lake effect snowfall map is the one that you're probably the most familiar.

CLASSIC Lake Effect Snowfall Map

The second example is LAKE EFFECT with a WEST WIND

Band or pockets of lake effect snow in this example usually hug the shoreline and are partially enhanced by the elevation. The fetch off of the lake is more significant for eastern areas. The secondary snowbelt usually doesn't push as far south.

NORTHWEST WIND: Lake Effect Snow Map

The third example is LAKE EFFECT with a NORTH WIND

This example can be very tricky because not only are you forecasting snowfall from the moisture in Lake Erie, north winds will carry moisture from LAKE HURON too! The Lake Huron component isn't as large as Lake Erie but it can be significant. The fetch isn't as far (distance from Canadian shoreline is ~50 miles) Yet with some instability created by BOTH lakes due to the cold air/milder lake, snow bands/pockets can stretch into Akron, Canton and even northern Tuscarawas county.

This example seems like a bit of a stretch as there are many components necessary for this to happen. Back in the winter of 2009-2010, we had several lake events that mirrored this example exactly.

Its easy to see how lake effect can and does change on a moments notice as the winds shift.A good understanding of the wind component is essential in
forecasting lake effect. Yet even the most comprehensive high
resolution data from the surface and aloft plugged into a very high
resolution computer model projection will never be exact.

No wonder we can have one location with 2 inches of snow and a few miles down the road, the amounts can reach 15"! That's why we use snowfall ranges.